
MOTOROLA
MC68330 USER’S MANUAL
3- 3
associated with read or write bus cycles. The function code signals are valid while
AS is
asserted.
Function codes (see Table 3-2) can be considered as extensions of the 32-bit address that
can provide up to eight different 4-Gbyte address spaces. Function codes are
automatically generated by the CPU32 to select address spaces for data and program at
both user and supervisor privilege levels, and a CPU address space for processor
functions. User programs access only their own program and data areas to increase
protection of system integrity and can be restricted from accessing other information. The
S-bit in the CPU32 status register is set for supervisor accesses and cleared for user
accesses to provide differentiation. Refer to 3.4 CPU Space Cycles for more information.
Table 3-2. Address Space Encoding
Function Code Bits
Address Spaces
210
0
Reserved (Motorola)
0
1
User Data Space
0
1
0
User Program Space
0
1
Reserved (User )
1
0
Reserved (Motorola)
1
0
1
Supervisor Data Space
1
0
Supervisor Program Space
1
Supervisor CPU Space
3.1.3 Address Bus (A31-A0)
The address bus signals are outputs that define the address of the byte (or the most
significant byte) to be transferred during a bus cycle. The MC68330 places the address on
the bus at the beginning of a bus cycle. The address is valid while
AS is asserted.
3.1.4 Address Strobe (
AS)
AS is an output timing signal that indicates the validity of an address on the address bus
and of many control signals.
AS is asserted approximately one-half clock cycle after the
beginning of a bus cycle.
3.1.5 Data Bus (D15-D0)
The data bus is a bidirectional, nonmultiplexed, parallel bus that contains the data being
transferred to or from the MC68330. A read or write operation may transfer 8 or 16 bits of
data (one or two bytes) in one bus cycle. During a read cycle, the data is latched by the
MC68330 on the last falling edge of the clock for that bus cycle. For a write cycle, all 16
bits of the data bus are driven, regardless of the port width or operand size. The MC68330
places the data on the data bus approximately one-half clock cycle after
AS is asserted in
a write cycle.
3.1.6 Data Strobe (
DS)